A conventional speaker system 200 having a circuit utilized in a mobile information terminal, such as a cellular phone, computerized personal organizer, handset, voice recognition device, voice memory device, computer, etc., each driven by a battery is shown in FIG. 6. A conventional speaker system 200 is expected to always generate a high and stable output, even when a battery power supply receives noise, the output level of the battery decreases due to deterioration with age, and when electric energy varies throughout such a device.
In conventional speaker system 200, an analog music signal Sin is output from an audio digital analog converter (audio DAC 1), and is input to a positive signal input terminal of an operational amplifier 2, that performs non-inverted amplification via a condenser C and a resistance r1. A signal ground SG is also input to the positive signal input terminal via a resistance r2. A prescribed regulator (not shown) generates signal ground SG. During amplification by the operational amplifier 2, a value of the signal ground SG defines a center amplitude of vibration of the analog music signal Sin. The signal ground SG can be set to be half the driving voltage.
For example, a half-voltage of a battery power source VBAT can be set as the signal ground SG using a resistance division circuit. However, the signal ground SG fluctuates in such a situation due to change in an output of the battery power supply VBAT. The change in the output is typically caused by noise, and changes in consumption of power in the device. As a result, a signal output from the operational amplifier 2 is unstable.
Further, the signal ground SG is simultaneously input to positive signal input terminals of operational amplifiers 3, 4, and 5.
A non-inverted amplification signal output by the operational amplifier 2 is input to a negative input terminal of operational amplifier 3, forming an inverted amplifier together with resistance r4 via resistance r3. The non-inverted amplification signal is simultaneously input to a negative input terminal of operational amplifier 5, forming a non-inverted amplifier together with resistance r8, via resistance r7. An inverted amplification signal output from operational amplifier 3 is input to a negative input terminal of operational amplifier 4 that performs inversion amplification via a resistance r5.
The battery power supply VBAT outputs a voltage of approximately 4.2V when fully charged. Thus, a constant voltage Vcc of approximately 3.0 volts is obtained by stepping down the above-mentioned power supply voltage VBAT using regulator 7, and is supplied as a driving voltage to operational amplifiers 2 and 3. The battery power supply VBAT is supplied as driving voltages to operational amplifiers 4 and 5, bypassing the regulator 7 in order to enhance an output of the speaker 6.
The speaker 6 sounds in accordance with signals output from the operational amplifiers 4 and 5, with their phases being deviated from each other by 180° as shown in FIGS. 1 and 6. Since the signal ground SG input to the positive signal input terminals of the operational amplifiers 4 and 5 is typically constant in speaker system 200, a sound output from the speaker 6 deforms along with deterioration of age of the battery power supply VBAT. Specifically, when a full charge voltage of the battery power supply VBAT is 4.2 volts, and the signal ground SG is 2.1 volts, the analog music signal Sin vibrates at the center of the signal ground SG as shown in FIG. 7A. However, when the battery power supply VBAT decreases down to 3.2 volts and the signal ground SG maintains 2.1 volts, the waveform of the analog music signal Sin is cut off at a portion of a positive phase higher than 3.2 volts and is deformed as shown in FIG. 7B.
Accordingly, the signal ground SG can be initially set to 1.6 volts in order to avoid the waveform from deforming as shown in FIG. 7C. However, when the battery power supply VBAT does not deteriorate at all, the negative phase is partially cut off and becomes deformed as shown in FIG. 7D. When such deformed waveforms of FIGS. 7B and 7D are utilized, the speaker decreases acoustically.